Space based planetary imagers are useful for remote sensing of atmospheric compositions, crop assessments, weather prediction and other types of monitoring activities. Monochromatic and multispectral satellite-based, remote sensors are able to measure properties of the atmosphere above the earth, when their detector arrays are properly calibrated for radiometric response.
A method of calibrating the radiance measured by these remote sensors is to create a reference radiation using a known source of spectral radiance, such as the sun. The radiation from the sun may be used as a reference signal to a diffusive reflector which, in turn, may provide a known radiance to a remote sensor for calibrating its detector arrays.
The output of the detector arrays may be measured as the remote sensor receives the known reflected energy from the diffusive reflector. This radiance calibration method provides sufficient information to correctly measure and calculate other types of radiance incident on the remote sensor during normal operation, such as radiance from views of the earth or other targets of interest.
The spectral characteristics of a diffusive reflector, or diffuser panel, however, may change with time due to degradation of the diffuser panel. Since the diffuser panel is employed as the reference source, any change, i.e., degradation of the diffusive surface material, results in a distortion in the measurement of the remote sensor.
Other diffusers, such as transmissive diffusers (for example screen or pinhole arrays) also have disadvantages. Screens or pinhole arrays have geometries that may cause undesired diffraction effects. Screens are also difficult to calibrate over ranges of angles, due to the three-dimensional nature of the screens, which may cause internal shadowing. Furthermore, pinholes are subject to clogging from extraneous minute particles.
The present invention provides a diffuser, referred to herein as a de-focusing optic, for an earth viewing solar wavelength spectrometer (such as the Climate Absolute Radiance and Refractivity Observatory (CLARREO)) so that it may stare straight at the sun without saturating. Moreover, the use of convolution integrals by the present invention allows direct in-flight measurements of the de-focused spectrometer's spectral throughput. As will be explained, the sun may thus be used as a calibration target for the spectrometer.